In melting a solid aluminum charge it is known to introduce the charge into a body of molten aluminum which is preferable to heating the dry charge since better heat transfer is achieved this way and since high rates of oxidation can result from exposure of the solid aluminum surfaces to the oxidizing influence of combustion gases. It is also known to utilize a circulating system of hot molten metal to melt aluminum wherein heating is accomplished in one compartment or zone and the heated metal is circulated to a second compartment or chamber where the charge is introduced. These systems offer the promise of much higher melting rates and reduced melt loss and have recently been developed to a state where they reliably increase melt rates while reducing melt losses by oxidation.
With the increased emphasis on environment controls and energy conservation there has been an increasing desire to recycle aluminum scrap and particularly aluminum can scrap. However, the economic considerations in remelting large amounts of aluminum can scrap can become disadvantageous in view of substantial melt losses which, to date, have not been completely alleviated even in recirculating melting systems. One particular problem encountered in melting can scrap in addition to those normally associated with remelting relatively clean scrap material is that the can scrap, having gone through the cycle of packaging, consumer use, scrap collection, and the like, is highly contaminated. Typical contaminants include food and other residues from the can container contents, paint residues and dirt residues. Troublesome contaminants include titanium dioxide from the paint pigments employed in decorating aluminum cans and silicon dioxide and calcium oxide, obvious contents of dirt. In addition there are other elements introduced in recycling cans but the above-mentioned oxides, especially the titanium and silicon oxides, are particularly troublesome. The contaminants mentioned can be present in significant amounts, typically 1 to 2% TiO.sub.2 and 2 to 4% SiO.sub.2 based on the total weight of a can scrap charge, and since they are chemically reduced by molten aluminum, or magnesium often present in molten aluminum, can introduce elemental silicon and titanium into the melt. These contaminants are typically present in both lacquered and delacquered can scrap or other sheet scrap. Thus considering the case where titanium is involved, the titanium level in the melt can rise to 0.1% as a result of the reduction of titanium dioxide whereas many alloys have the titanium level controlled to less than 0.05 percent It further being remembered that titanium is often introduced just prior to casting as a grain refiner, the problem becomes further complicated in that titanium must be held at a level sufficiently below the maximum limit to admit these last minute additions without exceeding that limit. A similar situation applies to silicon in that while aluminum alloys have a greater tolerance for silicon than for titanium, exceeding that tolerance introduces problems with respect to composition control. As silicon and titanium are both difficult to remove by normal fluxing treatments, the choice in the industry has been to simply counteract their contaminating effect by the addition of copious amounts of a purer or cleaner grade of aluminum metal. However, this approach is somewhat self-defeating in that typically two to four times as much make-up metal must be added to counteract the contaminating effect of a given can scrap charge with its titanium and silicon contaminant introduction effect. Remembering the objectives in recycling scrap, the self-defeating aspect becomes immediately apparent.
In addition to the titanium and silicon dioxide problems the introduction of a more or less dirty can scrap has disadvantages from the standpoint of developing excessive skim. In addition to the metal lost in association with the skim, excessive skim can be disastrous in a recirculating melting system and cut its efficiency by almost 50 percent and even seriously damage equipment.